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      Sitosterolemia: Diagnosis, Metabolic and Hematological Abnormalities, Cardiovascular Disease and Management

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          Abstract

          Sitosterolemia is a recessive inherited metabolic disorder of unknown prevalence, characterized by increased levels of plasma plant sterols. It is caused by 28 and 31 variants in ABCG5 and ABCG8 genes, respectively, and is characterized by a predisposition to hyperabsorption and accumulation of toxic levels of plant sterols in plasma. Its clinical picture is extremely heterogeneous. The main clinical features are tendinous and cutaneous xanthomas, arthritis or arthralgia, premature cardiovascular disease and atherosclerosis. These characteristics are shared with familial hypercholesterolemia (FH), making it possible for sitosterolemia to be misdiagnosed as homozygous FH, especially in pediatric patients. In such cases, a specific chromatography-based laboratory method is essential to differentiate sitosterol and cholesterol. Hematological abnormalities (hemolytic anemia and macrothrombocytopenia) may be present in 25-35% of patients, in whom it is usually associated with the main clinical features, as occurs in the 70% of the cases. In this context, the peripheral blood smear is essential and reveals giant platelets and stomatocytes. Only 21 causative variants in ABCG5/ABCG8 are associated with macrothrombocytopenia. Most physicians still do not recognize these hematological abnormalities or relate them to sitosterolemia. Patients may suffer long-term misdiagnosis of immune thrombocytopenia and be at high risk of receiving harmful therapies or of not benefitting from a low-cholesterol diet and/or from the gold standard treatment with ezetimibe. This drug reduces the levels of plasma plant sterols, provokes regression of xanthomas, and can alleviate hematological abnormalities. Finally, to identify genetic defects, recent advances in high-throughput sequencing, especially in the use of targeted sequencing of pre-specified genes, have begun to be incorporated in the first-line approach in the field of genetic disorders.

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          Most cited references52

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          Accumulation of dietary cholesterol in sitosterolemia caused by mutations in adjacent ABC transporters.

          In healthy individuals, acute changes in cholesterol intake produce modest changes in plasma cholesterol levels. A striking exception occurs in sitosterolemia, an autosomal recessive disorder characterized by increased intestinal absorption and decreased biliary excretion of dietary sterols, hypercholesterolemia, and premature coronary atherosclerosis. We identified seven different mutations in two adjacent, oppositely oriented genes that encode new members of the adenosine triphosphate (ATP)-binding cassette (ABC) transporter family (six mutations in ABCG8 and one in ABCG5) in nine patients with sitosterolemia. The two genes are expressed at highest levels in liver and intestine and, in mice, cholesterol feeding up-regulates expressions of both genes. These data suggest that ABCG5 and ABCG8 normally cooperate to limit intestinal absorption and to promote biliary excretion of sterols, and that mutated forms of these transporters predispose to sterol accumulation and atherosclerosis.
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            Beta-sitosterolemia and xanthomatosis. A newly described lipid storage disease in two sisters.

            Although the usual diet may contain 150-250 mg of plant sterols, chiefly beta-sitosterol, only trace amounts of these sterols have heretofore been found in human or animal blood and tissues. We now report elevated plant sterol levels in the blood and tissues of two sisters with extensive tendon xanthomas but normal plasma cholesterol levels. Besides beta-sitosterolemia and xanthomatosis, no other physical, mental, or biochemical abnormalities were detected.Repeatedly, the plasmas of the two sisters have contained 27.1 and 17.7 mg/100 ml of beta-sitosterol, 9.7 and 8.2 mg/100 ml of campesterol, and 0.5 and 0.5 mg/100 ml of stigmasterol, respectively. These plant sterols constituted 15.6 and 11.3% of the total plasma sterols. Some 60% of the plasma beta-sitosterol and campesterol was esterified; the measurable stigmasterol was entirely unesterified. The transport of the plasma beta-sitosterol and campesterol was largely in low density lipoproteins (76 and 83%, respectively). High density lipoproteins carried the remainder. Plant sterols were barely detectable in the very low density lipoprotein fraction. Only trace amounts of stigmasterol could be detected in the low density and high density lipoprotein fractions. The plant sterol content of the red blood cells averaged 12-13 mg/100 ml packed cells or about 13% of the total sterols. Two tendon xanthoma biopsies with the usual high concentration of cholesterol had 36.7 and 4.0 mg of plant sterols/g dry wt, of which 25.7 and 2.9 mg were beta-sitosterol, entirely in the free form. Plant sterols were also found in adipose tissue (0.2 mg/g wet wt) and in skin surface lipids (3.2 mg/g of lipid). The intestinal absorption of beta-sitosterol in both the patients, measured by two techniques, indicated greatly increased absorption of this sterol (about 24 and 28% in the patients L. H. and R. H., respectively, normal absorption being <5%). We suggest that increased absorption of beta-sitosterol must be considered as one cause of this disease. The reason for the extensive xanthomatosis in these two patients remains unknown. Perhaps in some way plant sterols initiated the development of xanthomas with otherwise normal plasma cholesterol levels. Clinical atherosclerosis has not yet occurred. The occurrence of beta-sitosterolemia in these two sisters with un-affected parents suggests an inherited recessive trait.
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              Identification of a gene, ABCG5, important in the regulation of dietary cholesterol absorption.

              The molecular mechanisms regulating the amount of dietary cholesterol retained in the body, as well as the body's ability to exclude selectively other dietary sterols, are poorly understood. An average western diet will contain about 250-500 mg of dietary cholesterol and about 200-400 mg of non-cholesterol sterols. About 50-60% of the dietary cholesterol is absorbed and retained by the normal human body, but less than 1% of the non-cholesterol sterols are retained. Thus, there exists a subtle mechanism that allows the body to distinguish between cholesterol and non-cholesterol sterols. In sitosterolemia, a rare autosomal recessive disorder, affected individuals hyperabsorb not only cholesterol but also all other sterols, including plant and shellfish sterols from the intestine. The major plant sterol species is sitosterol; hence the name of the disorder. Consequently, patients with this disease have very high levels of plant sterols in the plasma and develop tendon and tuberous xanthomas, accelerated atherosclerosis, and premature coronary artery disease. We previously mapped the STSL locus to human chromosome 2p21 and further localized it to a region of less than 2 cM bounded by markers D2S2294 and D2S2291 (M.-H.L. et al., manuscript submitted). We now report that a new member of the ABC transporter family, ABCG5, is mutant in nine unrelated sitosterolemia patients.
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                Author and article information

                Journal
                Current Medicinal Chemistry
                CMC
                Bentham Science Publishers Ltd.
                09298673
                December 17 2019
                December 17 2019
                : 26
                : 37
                : 6766-6775
                Affiliations
                [1 ]Department of Hematology, Hospital Universitario de Salamanca-IBSAL-USAL, Salamanca, Spain
                [2 ]Seccio d'Errors Congenits del Metabolisme-IBC, Servei de Bioquimica i Genetica Molecular Hospital Clínic, IDIBAPS, CIBERER, Barcelona, Spain
                [3 ]IBSAL, IBMCC, CIC, Universidad de Salamanca-CSIC, Salamanca, Spain
                Article
                10.2174/0929867325666180705145900
                29984642
                ad6ea0f7-fe29-48dc-a157-2c2e806e06f9
                © 2019
                History

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